The present invention relates to a method of determining and adjusting the upper dead-centre position in piston engines, in which the position of the piston is measured by distance measurement.
It also relates to a device for determining and adjusting the upper dead-centre position in piston engines, in which the position of the piston is measured by means of an electrically operated sensor.
The tip clearance is one of the most import factors in engine design. The less tip clearance, the less fuel consumption and less emissions of so called drive house gases. Therefor, it would be of advantage to be able to determine and control the distance between the piston tip or head and the cylinder head during engine operation, and at different rpm and different loads and temperatures.
Based on measurements of the tip clearance under relevant operation conditions, calculation models describing the tip clearance that are more accurate than contemporary theoretic model studies, such as FEM, would be possible to achieve.
U.S. Pat. No. 4,147,054 discloses a method and an apparatus for determining the upper dead-centre position in piston engines. The determination of the dead-centre position is based on a distance measurement, in which the distance between a sensor and the piston is measured. The sensor is either a capacitive distance-measuring transmitter of conventional construction or an inductive distance-measuring transmitter of conventional construction.
The capacitive distance-measuring transmitter is very sensitive to changes in the dielectric in the gap between sensor and the measuring surface, i.e. the piston head. Because the dielectric constant in the combustion gases is difficult to determine and also will change during the combustion, it will be difficult to obtain acceptable values on the tip clearance by means of such a capacitive distance-measuring transmitter.
The inductive distance-measuring transmitter has the disadvantage of only being applicable for the measurement on ferromagnetic materials such as iron. Accordingly, it will not serve for the measurement of aluminium pistons. It is also sensitive to temperatures and pressures and need to be protected from the combustion chamber environment. For the measurement regions that are most likely, typically up to 2 mm, the inductive sensor requires slightly more space than would be desired.
It is one object of the invention to provide a method and a device that remedies the disadvantages of the methods and devices of prior art. The inventive method should promote engine design and engine adjustments based on a measurement of the upper dead centre position.
The device should be reliable in operation, relatively cheap to manufacture and easy to handle and insert in the engine the tip clearance of which is to be determined and controlled.
The object of the invention is achieved by means of the initially mentioned method, characterised in that the position of the piston head is determined by means of an eddy-current sensor.
Thereby, the piston head comprises an electrically conducting material, and that eddy currents are induced therein, preferably by means of generating a current in a coil in the eddy-current sensor. The impact of these eddy-currents on the reactance in a coil of the eddy-current sensor is determined.
Preferably the determination of the upper dead-centre position is performed under dynamic operating conditions of the engine.
The object of the invention is also achieved by means of the initially defined device, characterised in that the sensor is an eddy-current sensor.
The device preferably comprises a coil and means for inducing a current through said coil, said coil being positioned such that an eddy-current will be induced in the piston head as a result of it being subjected to a magnetic field from said coil.
The device also comprises a coil in which the reactance is affected by the eddy currents induced in the piston head, and the determination of the dead-centre position is based on how said reactance is affected by the eddy-current.
Preferably, the coil for inducing the eddy-current in the piston head and the coil the reactance of which is affected by the eddy-current are one and the same measurement coil, forming a part of the eddy-current sensor.
According to one embodiment the inventive device comprises at least one temperature indicator for indicating the temperature that the eddy-current sensor is subjected to. The at least one temperature indicator preferably comprises a reference coil connected to the measurement coil in a Wheatstone bridge.
Preferably, the eddy-current sensor is located in a bore arranged in the cylinder head of the engine.
The bore extends through the cylinder head such that the eddy-current sensor is removable from outside the mounted cylinder head via the bore. Thereby, if desired, the sensor can be easily removed and replaced by a plug.
Preferably, the device is connected to a means for affecting the compression in the engine based on the upper dead-centre position determined by means of the eddy-current sensor.
According to one embodiment, the means for affecting the engine compression comprises a means for displacing the cylinder part of the engine in relation to the crankcase part thereof.
Further, the engine comprises at least two cylinders, each provided with a dedicated eddy-current sensor, and the device comprises a means for affecting the engine compression individually for each cylinder.
The invention also comprises a device for adjusting the combustion chamber volume and the compression in the combustion chamber of a piston engine, characterised in that a cylinder part of the engine is displaceable in relation to a crankcase part thereof. Thereby, the cylinder part is displaceable in relation to the crankcase part along a common sliding plane between cylinder part and crankcase part. Preferably, the cylinder part comprises a first flange and the crankcase comprises a corresponding second flange arranged to be in engagement with the first flange, and the device comprises a means for displacing the cylinder part in relation to the crankcase part through a sliding movement between said flanges. The displacement between cylinder part and crankcase part is preferably based on the determined tip clearance or on an algorithm based on the determined tip clearance.
Further features and advantages of the present invention are presented in the following detailed description of preferred embodiments of the invention and in the appended claims.